1. FIELD OF THE INVENTION
The present invention relates generally to one-shot circuits (monostable multivibrators) and more particularly to a one-shot circuit for tracking the period of a voltage-controlled oscillator (VCO).
2. BACKGROUND ART
One-shots are often used when a pulse of a controlled duration is required. One-shots are triggered by a rising or falling edge of an input signal. The input pulse must have a minimum width, but may be longer or shorter than the output pulse. The duration of a pulse generated by one-shot is often controlled by selecting external components, such as resistors and capacitors. Such components often have fixed values, preventing the adjustment of one-shot duration to match other circuit parameters. An example of circuit using a one-shot is given by U.S. Pat. No. 5,124,669, issued to Michael J. Palmer and Richard G. Yamasaki on Jun. 23, 1992 and incorporated herein by reference.
It may be desirable to use a one-shot in conjunction with a VCO. A VCO provides an output signal that varies in frequency in response to a control voltage. Since the frequency variability of a VCO results in variation of the period of the output signal, the duration of a one-shot used in conjunction with the VCO must be adjusted to maintain the appropriate proportional relationship between VCO period and one-shot duration.
In the prior art, a voltage reference is used to control the VCO in a phase-locked loop (PLL) used to extract clock timing information from an asynchronous data stream. The same voltage reference is also used to control the period of a one-shot. If the VCO is running at its center frequency, the one-shot duration is quite accurate, but if the VCO is operating at a frequency different from its center frequency, inaccuracies develop in the one-shot frequency. The error in the one-shot frequency tracks the percentage difference of the VCO frequency from its center frequency.
FIG. 1 illustrates a prior art circuit for controlling the duration of a one-shot pulse. FIG. 1 illustrates data recovery PLL circuit 101 and frequency reference PLL circuit 102. In the prior art, data recovery PLL circuit 101 and frequency reference PLL circuit 102 have been implemented as physically separate devices, typically in separate integrated circuits, preventing effective communication of signals between the circuits.
Data recovery PLL circuit 101 includes 1/3 cell delay one-shot 103, phase detector 104, CP block 105, loop filter 106, VCO control block 107, VCO 108, read reference clock (RRC) pulse width one-shot 109, write precompensation one-shot 110, inputs RD, RD* and VREF, Outputs VCOOUT and VCOOUT* and nodes 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161,162, 163, 164, 165, and 166.
Input RD is coupled to node 151, which is coupled to an input of 1/3 cell delay one-shot 103. Input RD* is coupled to node 152, which is coupled to another input of 1/3 cell delay one-shot 103. Input VREF is coupled to node 53, which is coupled to yet another input of 1/3 cell delay one-shot 103, to an input of RRC pulse width one-shot 109 and to an input of write precompensation one-shot 110. One output of 1/3 cell delay one-shot 103 is coupled to node 154, which is coupled to an input of phase detector 104. Another output of 1/3 cell delay one-shot 103 is coupled to node 155, which is coupled to another input of phase detector 104. An output of phase detector 104 is coupled to node 156, which is coupled to an input of CP block 105. Another output of phase detector 104 is coupled to node 157, which is coupled to another input of CP block 105. Yet another output of phase detector 104 is coupled to node 158, which is coupled to yet another input of CP block 105. Yet another output of phase detector 104 is coupled to node 159, which is coupled to yet another input of CP block 105.
An output of CP block 105 is coupled to node 160, which is coupled to an input of loop filter 106. Another output of CP block 105 is coupled to node 161, which is coupled to another input of loop filter 106. An output of loop filter 106 is coupled to an input of VCO control block 107. Another output of loop filter 106 is coupled to another input of VCO control block 107. An output of VCO control block 107 is coupled to node 164, which is coupled to an input of VCO 108. An output Of VCO 108 is coupled to node 165, which is coupled to output VCOOUT and to an input of phase detector 104. Another output of VCO 108 is coupled to node 166, which is coupled to output VCOOUT* and to another input of phase detector 104.
Frequency reference PLL 102 includes phase detector and counter 111, CP block 112, loop filter 113, VCO control block 114, VCO 115, inputs FREF and FREF*, outputs FRF and FRF* and nodes 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178 and 179.
Input FREF is coupled to node 167, which is coupled to an input of phase detector and counter 111. Input FREF* is coupled to node 168, which is coupled to another input of phase detector and counter 111. An output of phase detector and counter 111 is coupled to node 169, which is coupled to an input of CP block 112. Another output of phase detector and counter 111 is coupled to node 170, which is coupled to another input of CP block 112. Yet another output of phase detector and counter 111 is coupled to node 171, which is coupled to yet another input of CP block 112. Still another output of phase detector and counter 111 is coupled to node 172, which is coupled to still another input of CP block 112. An output of CP block 112 is coupled to node 173, which is coupled to an input of loop filter 113. Another output of CP block 112 is coupled to node 174, which is coupled to another input of loop filter 113. An output of loop filter 113 is coupled to node 175, which is coupled to an input of VCO control block 114. Another output of loop filter 113 is coupled to node 176, which is coupled to another input of VCO control block 114. An output of VCO control block 114 is coupled to node 177, which is coupled to an input of VCO 115. An output of VCO 115 is coupled to node 178, which is coupled to output FRF and to an input of phase detector and counter 111. Another output of VCO 115 is coupled to node 179, which is coupled to output FRF* and to another input of phase detector and counter 111.
In frequency reference PLL circuit 102, a stable differential frequency reference signal provided across inputs FREF and FREF* appears across nodes 167 and 168 and at a differential input to phase detector and counter 111. Phase detector and counter 111 allow a desired VCO frequency to be synthesized by scaling the frequency reference signal present across inputs FREF and FREF*. Phase detector and counter 111 compares the input signal to the output of VCO 115, which is fed back through nodes 178 and 179 to phase detector and counter 111. Phase detector and counter 111 produces an output at nodes 169, 170, 171 and 172 representative of the phase difference between the input signal and the VCO output. CP block 112 includes a charge pump that receives pump up and pump down signals from phase detector and counter 111 via nodes 169, 170, 171 and 172. CP block 112 outputs differential pump currents at nodes 173 and 174.
Loop filter 113 receives its input from nodes 173 and 174. Loop filter 113 is a low-pass filter used to smooth out fluctuations in the input signal. It also helps maintain a fixed phase relationship between the input signal and the VCO output. Loop filter 113 outputs filtered differential control voltages at nodes 175 and 176.
VCO control block 114 receives its input from nodes 175 and 176 and translates the filtered differential control voltages into a VCO control signal. If the filtered phase difference signals indicate a lag of the VCO output relative to the input signal, the period of the VCO output is shortened to eliminate the lag. Thus, VCO control block 114 generates a VCO control signal to increase the frequency of the VCO so as to shorten the period of the VCO output. If, however, the filtered phase difference signals indicate the VCO output is leading the input signal, the period of the VCO output is lengthened to eliminate the lag. Thus, VCO control block 114 generates a VCO control signal to decrease the frequency of the VCO so as to lengthen the period of the VCO output. VCO control block 114 provides the VCO control signal to VCO 115 through node 177.
VCO 115 is an oscillator that produces an oscillating output signal having a frequency that is a function of a control signal. The control signal is provided through node 177, and output is provided to nodes 178 and 179. Typical VCOs produce a frequency that is proportional or inversely proportional to a control voltage. The output of VCO 115 is fed back to an input of phase detector and counter 111 to provide a phase locked loop circuit.
In data recovery PLL circuit 101, an incoming raw data signal is provided to nodes 151 and 152 front inputs RD and RD*. The incoming data signal is delayed by one-third cell delay one-shot 103. The delay provided by one-third cell delay one-shot 103 is dependent upon a control voltage provided at node 153. One-third cell delay one-shot 103 outputs the delayed data signal to nodes 154 and 155.
The delayed data signal from nodes 154 and 155 is applied to a PLL including phase detector 104, CP block 105, loop filter 106, VCO control block 107 and VCO 108. Phase detector 104 compares the phase of the output of VCO 108 at nodes 165 and 166 with the delayed data signal from one-third cell delay one-shot 103 at nodes 154 and 155. Phase detector 104 outputs signals at nodes 156, 157, 158 and 159 representative of the relative phase difference between the inputs. The phase detector outputs, which include pump up and pump down signals, are applied to CP block 105. CP block 105 includes a charge pump which produces differential pump current outputs at nodes 160 and 161. Loop filter 106 receives its input from nodes 160 and 161 and performs low pass filtering on the signals to reduce the PLL's sensitivity to transient phase jitter and other spurious phenomena.
Loop filter 106 provides outputs at nodes 162 and 163. The lowpass-filtered differential control voltages at nodes 162 and 163 are applied to VCO control block 107, which determines whether the VCO is in phase with or is leading or lagging the PLL input signals present at nodes 154 and 155. If the VCO is in phase with the input signals, VCO control block 107 does not alter the characteristics of VCO 108. However, if VCO 108 produces outputs at nodes 165 and 166 that lag the input signals, VCO control block 107 adjusts a VCO control voltage at node 164 to increase the frequency of VCO 108 so as to bring VCO 108 back into phase with the input signal. If VCO 108 leads the input signals, VCO control block 107 adjusts a VCO control voltage at node 164 to reduce the frequency of VCO 108 so as to bring VCO 108 back into phase with the input signals.
VCO 108 is a voltage controlled oscillator that is controlled by the control input present at node 164 and that produces outputs at nodes 165 and 166. Nodes 165 and 166 are coupled to outputs VCOOUT and VCOOUT* and to the inputs of phase detector 104.
Input VREF is applied to node 153, which is coupled to inputs of one-third cell delay one-shot 103, RRC one-shot 109 and write precompensation one-shot 110. Input VREF is used to control the delay provided by these one-shots to attempt to match the frequency at which the system is operating. If the system is operating at a lower frequency, the delay must be lengthened to match the longer period associated with the lower frequency. If the system is operating at a higher frequency, the delay must be shortened to match the shorter period associated with the higher frequency.
In FIG. 1, input VREF is dependent upon the center frequency of VCO 108. Since input VREF is used to control the delay of one-shots 103, 109 and 110, the delay of these one-shots is also dependent upon the center frequency of VCO 108. Since a PLL is typically capable of locking over a range of frequencies, not just at the center frequency, an error may exist in the delay of one-shots 103, 109 and 110 if VCO 108 is operating at a frequency different from its center frequency. Thus, system performance may be diminished when the prior art system is employed.
FIG. 2 illustrates a prior art circuit for controlling one-shot delays. FIG. 2 is similar to FIG. 1. FIG. 2 illustrates data recovery PLL circuit 201 and frequency reference PLL circuit 102. Frequency reference PLL circuit 102 of FIG. 2 is identical to frequency reference PLL circuit 102 of FIG. 1. Data recovery PLL circuit 201 of FIG. 2 is similar to data recovery PLL circuit 101 of FIG. 1. However, whereas the system of FIG. 1 includes one-third cell delay one-shot 103, RRC one-shot 109 and write precompensation one-shot 110 receiving input VREF through node 153 and VCO control block 107 passing a control signal through node 164 to VCO 108, the system of FIG. 2 has VCO control block 107 passing a control signal through node 253 to VCO 108 and to one-third cell delay one-shot 103 and RRC one-shot 109. Input VREF is not present in FIG. 2. Also, VCO control block 107 has an additional output for providing an additional control signal through node 280 to conversion block 216. Conversion block 216 converts the additional control signal at node 280 into a control voltage that may be used to control a one-shot. Conversion block 216 produces an output node 281 that is received by write precompensation one-shot 110.
The delays provided by one-third cell delay one-shot 103 and RRC one-shot 109 are dependent upon the control signal provided at node 253. The delay provided by write precompensation one-shot is dependent upon the control signal present at node 280. The control signals at nodes 253 and 280 are provided by VCO control block 107. Thus, the output pulse durations of one-third cell delay one-shot 103, RRC one-shot 109 and write precompensation 110 are all controlled by VCO control block 107. Since VCO control block 107 produces outputs which determine VCO frequency variation from the VCO center frequency, the one-shot pulse durations track the VCO frequency variation rather than the VCO center frequency. However, VCO frequency variation may not always represent a corrective action to ensure locking of the PLL onto the input signal. VCO frequency variation can result from spurious phase jitter in the input data. Such phase jitter in the input signal can result in jitter of the outputs of phase detector 104, which can in turn cause jitter of the outputs of VCO control block 107, leading to jitter in the duration of the one-shot output pulses.
Because of the sensitivity to input data jitter, system performance may be diminished when the prior art system is employed.
Thus, there is a need for a one-shot control circuit that maintains accuracy over the entire PLL locking range and that is immune to data jitter.